The bulk storage of heat or coolness at certain temperature levels has many applications, such as in solar heating of buildings at 85.degree. to 120.degree. F., in solar Rankine engines or absorption refrigeration machines at 200.degree. to 250.degree. F., in off-peak hour operation of air conditioners at 30.degree. to 60.degree. F., in off-peak hour operation of refrigeration plants at -20.degree. to +20.degree. F., etc. The heat storage materials used, except in the case of water at 32.degree. F., must be carefully mixed in certain proportions with special equipment and techniques, and must be kept away from materials that will corrode. Such heat storage materials are bulky and heavy to transport, and must be used in contact with large area heat exchange devices because of the poor thermal conductivity of these heat storage materials.
In order to minimize volume, weight, and cost, heat of fusion materials with change of phase between solid and liquid have been proposed, tested and tried experimentally because 7,000 to 12,000 BTU's per cubic foot can be stored within the above narrow temperature ranges, whereas if only a liquid phase is used, such as water, capacity is limited to 2,000 to 3,000 BTU's per cu. ft. or so. These heat of fusion materials, particularly the sodium and calcium salt hydrates must have provisions to prevent stratification.
Most prior designs have used air as the heat transfer medium. Such prior art designs have been very bulky due to the required volume of the air ducts and also have required multiple encapsulation because of the requisite multiple air passages of comparatively large cross-sectional area. In certain instances, the prior art has attempted to utilize liquid as the heat transfer medium; however, such prior art arrangements have been largely limited to the freezing of water in metallic tanks, plates or tubes. However, such metallic structures suffer from corrosion and cause galvanic action which can cause rapid deterioration of various component parts. Moreover, they are expensive and inflexible, subject to damage due to expansion of the phase change material, and are heavy and difficult to transport. Other suggestions have involved multiple encapsulation with its consequent high cost.
Such prior art multiple encapsulation techniques have generally used small containers whose walls are not insulated, because the heat transfer must occur through the container walls. The uninsulated small containers are inefficient since undesired heat loss or heat inflow can readily occur through the uninsulated walls during periods of storage of heat or coolness. This problem of inefficiency of the uninsulated container is augmented by multiple small containers, because they inherently have a relatively large surface-to-volume ratio. Moreover, during the transfer of heat energy into the encapsulated containers, the PCM begins melting near the uninsulated wall of each container. The interior region of the PCM is the last to melt. Consequently, during most of a heat storing sycle, the larger proportion of the stored energy is relatively close to the container wall. In other words, a non-uniform distribution of the stored energy often exists, with more being stored, on average, near the uninsulated container wall where it is relatively easily lost to ambient. Convection of the melted PCM adjacent to the uninsulated container wall carries away heat energy during storage periods.